Image sensor pixel unit with polarization response characteristic light trapping structure

Abstract

The invention provides an image sensor pixel unit with a polarization response characteristic light trapping structure and a preparation method of the image sensor pixel unit. The image sensor pixel unit comprises a semiconductor substrate, a multi-layer structure arranged on the substrate and a polarization response unit. The multi-layer structure comprises an anti-reflection layer, a filling and leveling layer and a micro-lens layer. The polarization response unit adopts an all-dielectric sub-wavelength grating based on a backscattering technology, and the polarization response unit and the deep trench isolation structure are synchronously formed by adopting the same process, so that photoelectric isolation between pixels is realized, and a polarization selection function is also realized. The anti-reflection layer is a multi-layer dielectric film stack, and interface reflection is effectively restrained. By optimizing the design of a polarization selection-transmission-convergence cooperative structure, polarization selection and light enhancement functions are integrated, an all-dielectric material system is adopted to be completely compatible with a CMOS (complementary metal oxide semiconductor) process, and quantum efficiency is remarkably improved while high-efficiency polarization detection is realized.

Description

Technical Field

[0001] This invention relates to the field of image sensor technology, and in particular to an image sensor pixel unit with a light-trapping structure having polarization response characteristics. Background Technology

[0002] Polarization is one of the important properties of light, carrying rich information such as the shape, material, and stress of an object's surface. Polarization imaging technology has broad application prospects in remote sensing, medical diagnosis, industrial inspection, and military target identification. Traditional polarization imaging systems typically achieve this by attaching discrete optical polarizers in front of ordinary image sensors. This approach has significant limitations: the system is bulky, complex in structure, and expensive. Furthermore, the polarizers introduce additional alignment errors and significant light energy loss, reducing the system's sensitivity and signal-to-noise ratio.

[0003] To advance the miniaturization and integration of polarization imaging technology, the industry has developed on-chip polarization image sensor technology, aiming to integrate micro / nano polarization elements with photodetectors. Currently, advanced solutions in this field include fabricating metal nanowire polarizers on the pixel surface. For example, Sony's publicly disclosed technology uses silver (Ag) nanowire structures. While such metal structures can achieve polarization selection, their technical approach has inherent drawbacks: the nanofabrication process for the precious metal silver is complex, and its compatibility with standard CMOS processes is low, resulting in high manufacturing costs and significant yield challenges; more importantly, metal materials exhibit strong intrinsic absorption losses in the optical band, which significantly reduces the quantum efficiency of pixels, and their structure itself lacks light field enhancement capabilities, thus failing to solve the core problem of weak signals in polarization imaging. In standard image sensors, backscattering (BST) technology is often used to create special morphologies to achieve the light-trapping effect, thereby enhancing light absorption and improving quantum efficiency. However, traditional deep trench isolation, BST light-trapping structures, and polarization functional units are designed and fabricated independently. This separation not only increases process complexity but also fails to achieve synergistic optimization of optical performance. Therefore, there is an urgent need in this field for an innovative pixel solution that can simultaneously address the three major challenges of polarization selection, light absorption enhancement, and process compatibility. Summary of the Invention

[0004] The purpose of this invention is to overcome the defects of the prior art and propose an image sensor pixel unit with a polarization response light trapping structure. It aims to provide a novel pixel structure that is highly compatible with CMOS technology and can simultaneously achieve polarization selection and light absorption enhancement. Through the innovative design of the polarization selection-transmission-convergence synergistic structure, it solves the technical problems of large absorption loss, complex process and difficulty in co-optimization with existing optical systems in traditional metal polarization structures.

[0005] 1. An image sensor pixel unit with a polarization-response light-trapping structure, comprising, from bottom to top, a light absorption layer, a polarization response layer, an anti-reflection layer, a leveling layer, and a microlens layer, characterized in that the polarization response layer, the anti-reflection layer, and the microlens layer constitute a polarization selection-transmission-convergence synergistic structure, used to achieve modulation of the polarization state of incident light, reflection suppression, and light convergence, thereby improving the absorption efficiency of the pixel unit for target polarized light and reducing crosstalk.

[0006] Furthermore, the light-absorbing layer is a semiconductor material layer, including at least one of Si, Ge, Ga2O3, indium gallium arsenide, and GaN; it has a deep trench isolation structure inside to define independent pixel regions with a depth of 2.5 μm, and the filling material includes at least one of SiO2, TiO2, Ta2O5, HfO2, and organic polymers.

[0007] Furthermore, the polarization response layer is an all-dielectric subwavelength grating channel based on backscattering technology. This grating channel is composed of alternating dielectric materials with widths of 0.2 μm and 0.1 μm, forming a specific periodic structure that selectively responds to incident light with a specific polarization direction. The all-dielectric subwavelength grating channel material is consistent with the dielectric material of the deep trench isolation structure, allowing both functional structures to be fabricated through the same process step.

[0008] Furthermore, the antireflective layer is a multilayer dielectric film stack structure, comprising two or more dielectric materials selected from SiO2, Si3N4, TiO2 and HaO2 stacked alternately; the deep trench isolation surface is deposited with a high refractive index metal and a dielectric material, wherein the high refractive index metal includes at least one of Ag, Au, Cu and Al, and the dielectric material includes at least one of SiO2, TiO2, Ta2O5 and HfO2.

[0009] Furthermore, the planarization layer is an organic polymer planarization layer, including at least one of polyimide (PI), acrylic resin, and benzocyclobutene (BCB).

[0010] Furthermore, the material of the microlens layer includes at least one of SiO2, polymethyl methacrylate (PMMA), and epoxy resin.

[0011] 2. A method for fabricating a pixel unit of an image sensor with a light-trapping structure exhibiting polarization response characteristics. The present invention provides the following technical solution:

[0012] S1) Fabrication of the polarization-responsive layer:

[0013] On a semiconductor substrate serving as the absorption layer, the deep trench isolation structure and the all-dielectric subwavelength grating are simultaneously formed through a single patterning process. This step specifically includes the following sub-steps:

[0014] S11: Composite Patterning. A composite pattern comprising a deep trench isolation region pattern and a fully dielectric subwavelength grating pattern is defined on the semiconductor absorption layer using a single lithography process. The grating pattern is composed of alternating trenches with a width of 0.2 μm and a depth of 0.8 μm and a width of 0.1 μm and a depth of 1 μm.

[0015] S12: Simultaneous Etching and Shaping. Using the photoresist pattern formed in step S11 as a mask, an anisotropic dry etching process is employed to etch the underlying semiconductor absorption layer. Through this etching, the patterns of the deep trench isolation structure and the all-dielectric subwavelength grating are simultaneously transferred to the absorption layer, thereby forming a polarization response layer that combines pixel isolation and polarization selection functions in one step.

[0016] S13: Dielectric layer deposition. Silicon dioxide is deposited on a semiconductor substrate that has completed some front-end processes using plasma-enhanced chemical vapor deposition.

[0017] S2) Preparation of the anti-reflective layer:

[0018] An anti-reflection layer is fabricated on the polarization-responsive layer structure using plasma-enhanced chemical vapor deposition (PECVD) or atomic layer deposition (ALD). Contact windows are then defined at the top of the deep trench isolation structure using photolithography. A metal layer is deposited using electron beam evaporation, followed by a lift-off process to form precise metal contacts. Finally, a SiO2 insulating layer is deposited on the copper electrode surface using PECVD.

[0019] S3) Preparation of the fill layer:

[0020] An organic polymer material is coated onto the anti-reflective layer using a spin coater to form a filler film. The sample is then placed in an oven or on a hot plate for thermosetting, which cross-links and cures the organic polymer, achieving flow leveling and ultimately forming a leveling layer with an optically smooth surface.

[0021] S4) Fabrication of the microlens layer:

[0022] On the filler layer, photoresist is uniformly coated by spin coating, and then exposed using a stepper lithography machine to form a cylindrical photoresist pattern arranged in a predetermined array on the surface of the filler layer. The substrate with the formed cylindrical pattern is placed in a hot plate or reflow oven and subjected to thermal reflow treatment in a temperature range of 120°C to 180°C, so that the cylindrical photoresist melts and forms a smooth spherical lens shape by means of surface tension. After cooling and solidification, a polymer microlens array is obtained.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] First, unlike existing technologies that use noble metal (such as silver) nanowire structures, this invention uses an all-dielectric material system (such as silicon dioxide, silicon nitride, etc.), and its core structure and fabrication process are highly compatible with standard CMOS process flows. This avoids the complex challenges of noble metal nanofabrication and utilizes mature and low-cost semiconductor processes, thus reducing manufacturing costs.

[0025] Secondly, this invention innovatively integrates polarization selection with a backscattering trapping structure. The all-dielectric subwavelength grating used fundamentally overcomes the inherent optical absorption loss problem of metallic polarization structures. While achieving high-efficiency polarization selection, the periodic arrangement and specific size design of this structure can synergize with the absorption layer to produce significant optical field localization and trapping effects, thus realizing the integration of "polarization selection" and "light absorption enhancement". Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of the structure of a pixel unit in an image sensor; Figure 2 This is a partial cross-sectional schematic diagram of the layout and light-trapping structure morphology of the deep trench isolation structure in the absorption layer of the present invention. Figure 3 This is a top view and a cross-sectional view of the filler layer of the present invention; Figure 4 This is a schematic diagram illustrating the implementation process of fabricating an image sensor pixel unit according to an embodiment of the present invention.

[0030] Explanation of symbols in the diagram:

[0031] 1. All-dielectric subwavelength grating; 2. Deep trench isolation structure; 3. Semiconductor absorption layer; 4. Dielectric isolation; 5. Metal isolation; 6. Multilayer dielectric film stack anti-reflection layer; 7. Microlens; 8. Filler layer. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0033] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0034] Example 1: This example discloses the specific structure of an image sensor.

[0035] Reference Figure 1 The image sensor pixel unit of this invention includes an absorption layer 1, a polarization response layer 2, an anti-reflection layer 3, a leveling layer 4, and a microlens layer 5. Wherein:

[0036] Absorption layer 1 uses a semiconductor material as the photoelectric conversion absorption layer, including at least one of Si, Ge, Ga2O3, indium gallium arsenide, and GaN. It serves as the base layer for the pixel units of the image sensor, providing both absorption and support. Furthermore, it incorporates a deep trench isolation structure to define independent pixel regions.

[0037] Polarization-responsive layer 2, serving as the core functional layer of the pixel unit, is an all-dielectric subwavelength grating based on backscattering technology. This grating is composed of alternating dielectric trenches with widths of 0.2 μm and 0.1 μm, forming a periodic micro / nano structure with polarization selectivity. Its material is consistent with the dielectric material filling the deep trench isolation structure in absorption layer 1, such as... Figure 2 As shown.

[0038] The antireflection layer 3, covering the polarization-response layer 2, is a multilayer dielectric film stack composed of alternating layers of low-refractive-index and high-refractive-index media, such as... Figure 3 As shown. It includes one or more of SiO2 (refractive index 1.46), Si3N4 (refractive index 2.0), high-refractive-index oxides TiO2 (refractive index 2.4-2.7), or HaO2 (refractive index 2.0). The deep trench isolation surface is deposited with a high-refractive-index metal and a dielectric material. The high-refractive-index metal includes at least one of Ag, Au, Cu, and Al, and the dielectric material includes at least one of SiO2, TiO2, Ta2O5, and HfO2.

[0039] A leveling layer 4 is formed on the anti-reflective layer 3. This layer is preferably made of an organic polymer material with high light transmittance, low internal stress and good flowability, including at least one of polyimide and benzocyclobutene.

[0040] The microlens layer (5) is located above the leveling layer 4. As the uppermost optical structure of the pixel unit, its core function is to efficiently converge the incident light and accurately point the converged light spot to the corresponding polarization response layer 2 below.

[0041] Example 2 discloses an embodiment for fabricating the above-mentioned image sensor pixel unit. The absorption layer is Si, the polarization response layer (deep trench isolation) and the all-dielectric subwavelength grating are SiO2, the antireflective layer is a stack of SiO2 and HaO2, the high refractive index metal is Cu, the dielectric material is SiO2, the leveling layer is polyimide, and the microlens layer is epoxy resin.

[0042] Reference Figure 4 The implementation steps of this example include the following:

[0043] Step 1, prepare a polarization-responsive layer, such as Figure 4 (b)

[0044] like Figure 4 (a) A silicon substrate is used as an absorption layer. Through a one-step photolithography and etching process, a deep trench isolation structure pattern and an all-dielectric subwavelength grating pattern are simultaneously defined on the silicon substrate. The grating is composed of alternating trenches with a width of 0.2 μm and a depth of 0.8 μm and a width of 0.1 μm and a depth of 1 μm. The deep trenches are etched to a depth of 2.5 μm into the silicon substrate. Subsequently, a layer of SiO2 is deposited on the entire structure using plasma-enhanced chemical vapor deposition to completely fill the deep trenches and simultaneously form a SiO2 layer on the surface. Finally, the SiO2 on the surface is removed by chemical mechanical polishing until the original grating structure on the surface of the silicon substrate is exposed. Thus, a deep trench isolation structure filled with SiO2 dielectric and an all-dielectric subwavelength grating composed of SiO2 are simultaneously formed, which together constitute the polarization response layer 2.

[0045] Step 2, prepare the anti-reflective layer, such as Figure 4 (c)

[0046] On the surface of the polarization-responsive layer 2, five alternating SiO2 (15 nm thick) / HfO2 (25 nm thick) stacked structures are deposited sequentially using atomic layer deposition (ALD) to form a multilayer antireflective film stack with a total thickness of 200 nm. Subsequently, a contact window is defined in the top region of the deep trench isolation structure using photolithography, and a 200 nm thick Cu metal layer is deposited using electron beam evaporation, followed by a lift-off process to form precise metal contacts. Finally, a SiO2 insulating layer is deposited on the copper electrode surface using plasma-enhanced chemical vapor deposition (PECVD), and a semi-cylindrical SiO2 rod-shaped structure with a radius of 100 nm is formed in the top region of the trench using reactive ion etching combined with nanosphere photolithography.

[0047] Step 3, prepare the leveling layer, such as Figure 4 (d)

[0048] On the surface of the composite structure after step 2, a polyimide precursor solution was spin-coated, achieving a precise thickness control of 0.4 μm by controlling the spin-coating speed at 5000-6000 rpm. Subsequently, an optimized curing process was performed in a nitrogen atmosphere: first, preheating and curing at 120℃ for 3 minutes to rapidly evaporate the solvent, then gradually increasing the temperature to 300℃ and maintaining it for 20 minutes to complete the final cross-linking and curing. This optimized thin-layer planarization process ensures complete coverage of the complex morphology of the semi-cylindrical SiO2 rods and metal electrodes at the top of the deep trenches while achieving excellent global planarization, providing a superior optical interface and mechanical support for subsequent microlens fabrication.

[0049] Step 4, prepare the microlens layer, such as Figure 4 (e).

[0050] On the planarized filler layer 4, epoxy resin photoresist is spin-coated to form a uniformly thick photoresist layer. Array exposure is then performed using a photolithography machine to form a cylindrical pattern array with a bottom radius of 1.0 μm on the filler layer surface. By controlling the thermal reflow process parameters (150℃, 90 seconds), the cylindrical structure is melted to form a spherical microlens with a height of 0.65 μm. Finally, cross-linking and curing are completed through full UV exposure and a 30-minute post-bake at 135℃, forming the epoxy resin microlens layer.

[0051] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention. The scope of protection claimed by the appended claims and their equivalents is defined.

Claims

1. An image sensor pixel unit with a light-trapping structure having polarization response characteristics, characterized in that, The structure includes an absorption layer (1), a polarization response layer (2), an anti-reflection layer (3), a leveling layer (4), and a microlens layer (5) arranged from bottom to top. The polarization response layer (2), the anti-reflection layer (3), and the microlens layer (5) constitute a polarization selection-transmission-convergence synergistic structure, which is used to modulate the polarization state of the incident light, suppress interface reflection, and converge light energy, thereby improving the absorption efficiency of the pixel unit for the target polarized light and reducing crosstalk.

2. The image sensor pixel unit with a polarization-response light-trapping structure according to claim 1, characterized in that, The absorption layer (1) is a semiconductor material layer used to realize photoelectric conversion. It has a deep trench isolation structure inside to define independent pixel areas. The deep trench isolation structure is filled with dielectric material.

3. The image sensor pixel unit with a polarization-response light-trapping structure according to claim 2, characterized in that, The semiconductor material includes at least one of Si, Ge, Ga2O3, indium gallium arsenide, and GaN; the dielectric material of the deep trench isolation structure includes at least one of SiO2, TiO2, Ta2O5, HfO2, and organic polymers.

4. The image sensor pixel unit with a polarization-response light-trapping structure according to claim 3, characterized in that, The polarization response layer (2) is an all-dielectric subwavelength grating based on a light-trapping structure, which includes backscattering technology and a standard light-trapping structure.

5. The image sensor pixel unit with a polarization-response light-trapping structure according to claim 4, characterized in that, The all-dielectric subwavelength grating is composed of periodically arranged dielectric microstructures, which contain the same dielectric material as the deep trench isolation structure, and the structural units constituting the period are greater than or equal to one type.

6. The image sensor pixel unit with a polarization-response light-trapping structure according to claim 1, characterized in that, The anti-reflection layer (3) is a multilayer dielectric film stack composed of one or more of SiO2, Si3N4, TiO2, and HaO2.

7. The image sensor pixel unit with a polarization-response light-trapping structure according to claim 1, characterized in that, The filler layer (4) is an organic polymer planarization layer that provides a planarized substrate for the microlens layer (5).

8. The image sensor pixel unit with a polarization-response light-trapping structure according to claim 1, characterized in that, The microlens layer (5) is made of silicon dioxide or resin material and is configured to direct the convergent incident light toward the corresponding polarization response layer (2) region below.

9. A method for fabricating a pixel unit of an image sensor with a light-trapping structure having polarization response characteristics, characterized in that, Includes the following steps: S1) Fabrication of a polarization-responsive layer: On the semiconductor substrate serving as the absorption layer (1), a deep trench isolation structure and a fully dielectric subwavelength grating consisting of alternating arrangements of 0.2 μm wide and 0.8 μm deep and 0.1 μm wide and 1 μm deep are simultaneously formed through the same patterning and silicon dioxide deposition process of one photolithography and one etching, so as to jointly constitute the polarization response layer (2). S2) Preparation of the anti-reflective layer: On the polarization response layer (2), dielectric films with different refractive indices are sequentially and alternately deposited by chemical vapor deposition or atomic layer deposition to form a multilayer dielectric film stack covering the entire structure; further photolithography and etching expose the top of the deep trench isolation structure, and deposit a stack of metal or dielectric materials. S3) Preparation of the fill layer: An organic polymer is spin-coated onto the anti-reflective layer (3) and a planarized filler layer is formed by a thermosetting process, including at least one of polyimide and benzocyclobutene; S4) Fabrication of the microlens layer: On the filler layer (4), the microlens layer (5) made of silicon dioxide or resin is formed by photolithography and reflow process.

10. The method according to claim 9, characterized in that: The metal mentioned in step S2) includes at least one of Ag, Au, Cu, and Al; the dielectric material includes at least one of SiO2, TiO2, Ta2O5, and HfO2.

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